Add command to get unique values / distinct count

[AliiiBenn]

xlforge lacks a command to analyze data distribution. Users cannot easily get:

• List of unique values in a column
• Count of distinct values
• Frequency distribution

This is needed for initial data exploration, e.g., understanding what types of movements exist in a movements log.

Labels: feature, priority:medium, effort:m

Example use case:

xlforge data unique movements.xlsx \"data export\" \"Type\"
# Output:
# Piger: 4521
# Charger: 8938